Stone Matrix Asphalt (SMA) has been used successfully in Europe for over 20

years to provide better rutting resistance and to resist studded tire wear.

Since 1991, the use of SMA has increased steadily in the United States. At

present, some states routinely use SMA even though a standard mixture design

procedure is not available. A mixture design procedure that provides guidance

on material properties, aggregate gradation, determination of optimum asphalt

content, and mixture properties is needed. This paper presents a mixture design

procedure for SMA mixtures developed by the National Center for Asphalt

Technology. Data for the development of the procedure was collected from a

laboratory study conducted with various samples of aggregates, fillers, asphalt

binders, and stabilizing additives. Compacted mixtures were tested to evaluate

the effects of aggregate structure, asphalt binder, and binder-fine aggregate

mortar. Specific conclusions from this study were: (1) The Los Angeles abrasion

loss showed good correlation with aggregate breakdown, (2) It appeared that the

3:1 or 2:1 flat and elongated particles provided much better classification for

the various aggregates than a 5:1 ratio, (3) The flat and elongated particle

ratio showed excellent correlation with aggregate breakdown, (4) In a SMA mix,

the percent passing the 4.75 mm sieve must be below 30 percent to ensure proper

stone-on-stone contact, (5) The percent passing the 0.02 mm sieve did not show

a correlation with mortar stiffness. However, the dry compacted volume, as

obtained from the Penn State test method, did show a good correlation with

mortar stiffness and can be utilized to characterize the shape of fillers.

Generally, a more angular filler tends to produce a higher air voids result in

this test, (6) In-place results from about 86 projects showed that very little

rutting has occurred in SMA pavements constructed in the United States since

1991. However, for the pavements with air voids falling below the 3 percent

range, some rutting was observed. (7) A VMA significantly lower than specified

VMA can be obtained due to aggregate breakdown. Hence, the mix designer must

consider aggregate type, compactor type and compactive effort along with the

gradation in meeting the required VMA criteria. Specifying a minimum asphalt

content can result in different requirements for aggregates with different

specific gravity, (8) Fifty blows of Marshall hammer were found to be

approximately equal to 100 revolutions of the Superpave gyratory compactor in

terms of resultant density. The Superpave gyratory compacter was found to

produce less aggregate breakdown than the Marshall hammer, (9) Fiber stabilizers

were found to be more effective in reducing draindown than polymer stabilizers.

However, mixes modified with polymer showed better resistance to rutting in

laboratory wheel tracking tests.